Demand for high performance communication networks capable of transporting multiple types of data, such as text, audio and video data, is on the rise. To carry greater amount of data over existing communication channels, such as fiber-optic communication channels, network carriers are increasingly using high bandwidth technologies, such as wavelength division multiplexing (WDM) and optical carrier (OC) level 192. Such communication networks rely upon high performance packet switches, such as asynchronous transfer mode (ATM) switches, frame relay switches and internet protocol (IP) routers which route the incoming packets to their desired destinations. Fiber-optic communication systems provide a number of advantages over conventional copper-based systems. Among such advantages are the ability to carry higher volume of information at greater speeds, and a reduced need for signal amplification when transferring signals over long distances
To utilize the high bandwidth capability of existing fiber optic communication channels, data is typically transmitted through many such channels through multiplexing. Two multiplexing methods exist, namely time division multiplexing (TDM) and frequency division multiplexing (FDM).
In accordance with the TDM technique, data bits associated with different channels are interleaved in the time domain to form a composite bit stream. For example, assume that each time slot is about 15 us for a single voice channel operating at 64 Kb/sec. Accordingly, five such channels may be multiplexed via the TDM technique if the bit streams of successive channels are delayed by 3 usec. Most telecommunication networks implement TDM for transfer of digital signals. A commonly known standard, referred to as synchronous optical network (SONET), defines a synchronous frame structure for transmitting signals using TDM.
In accordance with the FDM technique, the channels are positioned along the frequency domain with the carrier frequencies being spaced apart more than the channel bandwidth so as to inhibit channel spectra overlap. When FDM is implemented in optical domain, it is often referred to as WDM. A WDM system typically uses a number of optical channels each having an assigned channel wavelength. The optical signals in each channel are multiplexed to form a composite optical signal. The composite optical signal is transmitted and subsequently demultiplexed such that the received optical signal associated with each wavelength is routed to its destination.
In many applications, such as optical LANs, there is a need to route the optical signals associated with one or more optical channels to different destinations. One known technique for optical routing is commonly referred to as add/drop multiplexing. To perform add/drop multiplexing, the wavelength of the optical signal added to the network may need to be converted to a different value.
One conventional technique developed for converting the wavelength of an optical signal is to convert the optical signal to an electrical signal and then generate an optical signal with a different wavelength from the electrical signal. FIG. 1 is a simplified block diagram of a conventional optical-electrical wavelength converter 10, as known in the prior art. Wavelength converter 10 is shown as including an optical-to-electrical signal converter 12, an electrical control system 14, and an electrical-to-optical converter 16. Optical-to-electrical signal converter 12 is adapted to receive an optical signal having wavelength λ1 and convert it to an electrical signal E1. Electrical control system 14 receives electrical signal E1, and in response generates electrical signal E2. Electrical-to-optical signal converter 16 receives signal E2, and in response, generates an optical signal having wavelength λ2. Optical-to-electrical signal converter 12, electrical control system 14 and electrical-to-optical signal converter 16 are often required to operate at relatively high speeds, and thus are expensive and consume relatively high power.
To overcome some of the shortcomings of optical-electrical wavelength converters, all optical wavelength converters have been developed. FIG. 2 is a simplified block diagram of an all-optical wavelength converter 20 adapted to convert an incoming amplitude modulated optical signal at the wavelength λ1, to an optical signal at the wavelength λ2 with the same amplitude modulation, as known in the prior art. Optical wavelength converter 20 includes an integrated optical chip containing semiconductor optical amplifiers and a Michelson interferometer (SOA-MI) 22, an optical source 26 and an optical circulator 24. SOA-MI 22 receives amplitude modulated optical signal A having wavelength λ1 and a continuous-wave (CW) optical signal B at wavelength λ2, which is generated by CW laser 26, and which has wavelength λ2 through an optical circulator 24. As known to those skilled in the art, semiconductor optical amplifier and a Michelson interferometer 22 causes the occurrence of cross-phase modulation of the optical signal at wavelength λ2 due to the strong optical signal at wavelength λ1. In response, SOA-MI 22 generates optical signal C that has the same wavelength λ2 as signal B but whose amplitude follows the amplitude of signal A. The optical signal C comes out from 22 at the same port as the optical signal B goes in 22. The optical circulator 24 channels the optical signal C to the output.
The main purposes to convert wavelengths all in optical domain are to reduce power consumption and complexity to save cost. Prior art all-optical wavelength converters, such as that shown in FIG. 2, cannot achieve these goals. One of the figures of merit of a wavelength converter is the extinction ratio, which is directly related to the signal to noise ratio of a signal. In order to minimize the degradation of the signal to noise ratio, it is necessary to make the extinction ratio as large as possible. Unfortunately, Michelson interferometer is known to produce signals with poor extinction ratio due to fabrication tolerances and environmental effects. Therefore, in order to achieve extinction ratios good enough for telecommunication applications, active bias control is necessary. This bias controller contains high speed photonics and electronics that can detect and analyze the data carried by the output optical signal at the output of the optical wavelength converter. Such bias controllers are expensive and have large power consumption defeating the main purposes of low cost and low power consumption. Thus, a need continues to exist for an all optical wavelength converter that is low cost and more efficient.